As a method for purifying exhaust gas discharged from an internal combustion engine such as an automobile, a method using a monolithic (or honeycomb) support is generally used. This is obtained by applying a catalyst component on a monolithic structure as a molding member. The monolithic structure is obtained by rolling a molded article of a refractory inorganic oxide crystal such as cordierite or a metal thin plate, and has a flow path finely carved parallel to a gas flow direction. In a catalyst for internal combustion engine exhaust gas purification, in general, a wall surface of this flow path is thinly coated (support-coated) with a composition containing a catalyst active component, and a reaction of purifying exhaust gas passing at a high temperature proceeds.
As a catalyst for purifying exhaust gas from a gasoline automobile, a so-called three-way catalyst is used. This means a catalyst suitable for purifying exhaust gas generated by combustion with an engine approximately at a stoichiometric ratio. A stoichiometric ratio of a mixed gas formed of the air and a fuel is usually represented by an air-fuel ratio. The three-way catalyst contains rhodium, palladium, or platinum as a main active component, and was found about 35 years ago. Thereafter, technological development has been made by discovery of an oxygen storage material, improvement in a sensor technology, a processing speed of a computer, and engine controllability, improvement in durability of a catalyst, or the like. In recent years, a technology has advanced to such a level that exhaust gas having a lower concentration than that of a hazardous substance contained in the air is discharged under normal driving conditions. Meanwhile, the three-way catalyst uses a noble metal or a rare earth element. Therefore, resource-saving and an economic efficiency due to reduction in a use amount thereof has become one of major objects.
Examples of technical problems related to emission reduction include measures on discharge of a hazardous substance by engine start-up. In vehicle test driving such as a FTP test in the United States, a discharge amount of a regulated substance such as a hydrocarbon (HC) or a nitrogen oxide (NOx) in this region occupies a large proportion in the whole test cycle. Therefore, reduction in a hazardous substance in a so-called cold staring region is effective as measures for emission reduction.
Reasons why a discharge amount is large at the time of engine start-up are that combustion of a fuel in an engine tends to be incomplete and a catalyst does not act. The temperature of a catalyst substance has not reached an acting temperature region due to a low temperature, and therefore a purification reaction does not proceed easily. In order to promote temperature rising of a catalyst body at the time of engine start-up, such a contrivance as described below to shorten time before activation of a catalyst has been made. That is, the temperature of exhaust gas is raised by adjusting a fuel injection amount or combustion timing in an engine, and a catalyst is disposed in a position proximate to an engine.
Therefore, an important function required for a proximity catalyst is early activation at the time of engine start-up, and therefore a proximate position is generally advantageous. However, for example, a catalyst is exposed to a high temperature including reaction heat in normal use after activation, or is poisoned by a sulfur (S) component or a phosphorus (P) component contained in a fuel or an engine oil. Therefore, a catalyst desirably has high durability.
In response to a growing concern about greenhouse gas, regulation on greenhouse gas has been strengthened.
This is so-called fuel economy regulation or CO2 regulation. There is a trend to apply the regulation to greenhouse gas represented by nitrous oxide (N2O). It is considered that exhaust gas in combustion in an engine contains substantially no nitrous oxide. However, nitrous oxide is formed/generated secondarily from NOx (NO and NO2) which has passed through a monolithic catalyst. A mechanism of formation of nitrous oxide is described by such a reaction formula as follows. Here, exhaust gas hardly contains NO2, and therefore a reaction formula with NO is described.[Chemical Formula 1]2NO+CO→N2O+CO2  reaction formula 12NO+H2→N2O+H2O  reaction formula 2
On a surface of a noble metal, as a reaction formula between adsorption species,[Chemical Formula 2]NO(ad.)→N(ad.)+O(ad.)  reaction formula 3N(ad.)+NO(ad.)→N2O(ad.)→N2O  reaction formula 4
is considered. In the above formula, (ad.) indicates a reaction precursor adsorbed on a surface of metal. Therefore, considering the above reaction formulae 3 and 4, a generation mechanism of nitrous oxide can be understood.[Chemical Formula 3]N2O+H2→N2+H2O  reaction formula 5N2O+CO→N2+CO2  reaction formula 6
In the above reaction formulae, reaction formulae 1 and 2 indicate reaction formulae by which N2O is formed from NO and CO or H2. Reaction formulae 3 and 4 exemplify a reaction mechanism on a surface of a catalyst. It is known that a speed or a mechanism in these reactions depends on the kind of a catalyst element. In evaluation of light-off characteristics of a rhodium catalyst, a palladium catalyst, and a platinum catalyst, formation of nitrous oxide (N2O) can be observed mainly at a low temperature (100 to 300° C.). Reaction formulae 5 and 6 indicate reaction formulae by which nitrous oxide formed is purified. Therefore, in order to suppress discharge of nitrous oxide, there are two technical problems of suppressing formation of nitrous oxide and purification thereof. A palladium catalyst produces nitrous oxide easily, and it is considered that this reaction proceeds by reaction formula 4 above or the like. There is little precedent related art regarding suppression of nitrous oxide within a range in which the inventors have studied in the present field. Patent Literatures 1 and 2 are related art for separating a catalyst into regions, but dispose palladium in an uppermost stream region, and therefore do not suppress discharge of nitrous oxide easily.